The horizons of cancer biology have been expanded by the concept of cancer stem cells (CSC). In concept, tumors may arise from and continue to harbor a stem cell population which is responsible for ongoing tumor self renewal. This distinct population may hold the key to understanding the development of treatment resistance. Because of these factors, the study of cancer stem cells promises novel insights into the origins and treatment of cancer. The Ewing Sarcoma Family of Tumors (ESFT) represents a particular challenge. Its cell of origin is unknown. While initially very responsive to chemotherapy, the emergence of treatment resistance occurs in over 40%, usually with fatal results. ESFT are united by a chromosomal translocation fusing the EWS gene an ETS transcription factor. The prototype fusion, EWS/FLI11 occurs in over 85% of cases. Abundant evidence supports a crucial role for EWS/FLI1 in ESFT through its function as an aberrant transcription factor. My lab has recently uncovered a novel effect of EWS/FLI1 in enhancing expression of GLI1 in ESFT. GLI1 is the effector of the Hedgehog/GLI (HH/GLI) signaling pathway. This pathway has prominent roles in development and in many malignancies. It has been shown to contribute to the maintenance of stem cell phenotype in other tumor systems. This proposal will investigate the hypothesis that GLI1 upregulation by EWS/FLI1 is critical to maintaining a stem cell compartment in ESFT and whether this CSC compartment is resistant to treatment Since ESFT have been shown to harbor CD133 positive CSC, we will select CD133 positive fractions from ESFT cell lines and perform in vivo tumorigenesis assays. Validated CD133 sorted ESFT cell populations will be assessed by gene expression array to assess the expression of GLI1 and its targets. We will also assess for other potential stem cell markers. The role of GLI signaling in ESFT CSC will be tested by measuring the quantitative effects of GLI1 inhibition on the CSC fraction. Methods of inhibition will include both GLI1 specific shRNA and small molecule inhibitors of GLI signaling. We will also test the results of GLI inhibition on CSC function by measuring the effect of GLI inhibition on CSC tumorigenesis assays. We will also test the hypothesis that stem cell populations are associated with treatment resistance and whether GLI1 contributes to this. This will be tested using validated CD133 positive CSC populations generated in Aim 1. Using systems in place for the Ewings Preclinical Testing Lab, we will measure any differences in sensitivity to a wide range of relevant drugs in already characterized ESFT cell lines. These changes will be documented by high throughput in vitro assessment of chemosensitivity (DIMSCAN). PUBLIC HEALTH RELEVANCE: Ewing's Sarcoma is a deadly cancer of childhood, adolescence and young adulthood which takes the life of over 40% of those diagnosed. This work will detail the role of Gli1 signaling in maintaining cancer stem cells in Ewing's Sarcoma. If Gli1 is important in maintaining cancer stem cells, these treatment- resistant cells can be specifically targeted with drugs, enhancing our ability to cure patients.